A realistic model of the major pathways of intermediary metabolism in Tetrahymena pyriformis has been written. The model incorporates all known data on which enzymes are present and in which intracellular compartments they are localized. Equations have been written that allow one to compute the specific activity of every carbon atom of every metabolite in the system in the steady state for 14C-labeled input from any of 8 labeled substrates with which measurements are made. Cells grown under 4 different conditions - control, supplemented with L-propranolol, with glucose, or with glucose followed by a 5-hour period of anaerobiosis - were incubated for 1 hour with a suitable mixture of substrates so that all flasks were in the same metabolic state. Measurement of label incorporation into CO2, glycogen, lipid glycerol and fatty acids, RNA, glutamate, and alanine have been made for various known 14C-labeled substrates, and we are now ready to attempt to find a unique set of flux rates along all the pathways of intermediary metabolism which will account for the large amount of data we have acquired. Work is also beginning on applying this methodology to isolated rat hepatocytes.